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Page 30 | AlliedWare™ OS How To Note: VPNs for Corporate Networks
How to make voice traffic high priority
This is an optional enhancement to the configuration of the routers. It prioritises outgoing
voice traffic higher than other outgoing traffic on each VPN, to maximise call quality.
Use the configuration in this section if you expect your VPN client or branch office users will
be using VoIP over a VPN. The configuration consists of the following sections:
z "How to prioritise outgoing VoIP traffic from the headquarters router"
z "How to prioritise outgoing VoIP traffic from the branch office
1
router"
z "How to prioritise outgoing VoIP traffic from the branch office 2 router"
This enhancement prioritises outgoing voice traffic rather than incoming voice traffic,
because the link to the ISP is the most likely point of congestion.
We recommend you configure prioritisation on both peers in each VPN, not just on the
headquarters router, because the link from either peer to its ISP could become a point of
congestion.
Note: This enhancement only prioritises outgoing VoIP traffic at the routers you configure
it on. Of course, you cannot control the quality of service on routers in the Internet, such as
your ISP’s routers.
Classifying
VoIP traffic
In the following configurations, the router classifies voice traffic by checking packets’ IP DSCP
values. If the originating VoIP appliance does not mark packets with a DSCP value, you can
instead select the voice traffic by:
z classifying on the range of destination ports that the appliance uses for RTP (Real Time
Protocol—the protocol that carries voice data), or
z use the Dynamic Application Recognition (DAR) system, which dynamically determines
the ports
If you also need to classify the signalling traffic (call setup etc) and the signalling traffic is not
DSCP-marked, then create classifiers for the appropriate ports. For example, H.323 signalling
traffic uses TCP ports
1
720 and
1
72
1
, and SIP uses UDP port 5060. To create classifiers for
H.323 signalling packets, use the following commands:
create classifier=<id> tcpd=1720
create classifier=<id> tcpd=1721
For more information, including information about DAR, see the following:
z the “Software Quality of Service (QoS)” and “Generic Packet Classifier” chapters of the
Software Reference
z How To Configure Software QoS For Some Specific Customer Scenarios. This How To Note is
available from www.alliedtelesis.com/resources/literature/howto.aspx.